This invention relates to the formation of threaded holes in graphite panels and in particular to the repair of damaged threads in such panels.
A method presently in use to separate metal isotopes is the atomic vapor laser isotope separation (AVLIS) process wherein a beam of neutral metal atoms is irradiated by tunable lasers to ionize atoms of a desired isotope of the metal. In general, an AVLIS process is carried out in a vacuum chamber, with the metal (such as uranium) to be processed being put into an open crucible and heated from above by an electron beam sufficient to generate a substantial vapor pressure of the material from the surface of the electron-beam-heated melt. The gas-phase atoms are mostly in the lowest-lying electronic states and when irradiated by tunable lasers of the proper wavelength the atoms of the desired isotope of the metal will ionize. The ionized atoms are attracted to an electrically charged extractor while the beam of remaining neutral atoms in the gas phase continues unimpeded in its flow until it condenses and collects on a collector plate.
Many of the components in the AVLIS process are graphite panels and other shapes made of Stackpole 20--20, a high purity graphite, the panels being held in place by bolts that screw into threaded holes formed directly into the graphite. Periodically, the panels must be disassembled for inspection, for removal of the metal film resulting from the condensation of the hot gases on the panels, and for other normal maintenance. Such disassembly often causes damage to the threaded holes in the panels.
Previously, if the damage to the threads was enough so that bolts would not hold upon reassembly, the panels would be discarded. This is very costly since the panels are custom-made and come in various sizes and shapes, costing several thousands of dollars apiece. Because of the cost, replacement panels would not be kept in stock and several weeks would be required to fabricate a new panel.
Attempts have been made to repair such damaged threads in graphite panels by substantially enlarging the diameter of the hole and gluing a metal nut in the hole, the nut having a hole therethrough with threads of the desired pitch and diameter. This attempt proved unsatisfactory because the metal bolt and nut would freeze together when subsequently subjected to the high temperature (e.g., 1200.degree. C.) of an AVLIS process. Lubricants are available to prevent such freezing, but would be detrimental to the process in that the lubricants would evaporate into the vacuum environment and condense on the surfaces of the panels.
Another repair attempt is that shown in FIGS. 1 and 2 herein. In this case, the region of the panel surrounding the damaged threaded hole was drilled or reamed out to form a pocket 1 having a diameter slightly more than twice that of the damaged hole. A circular graphite plug 2 having an outside diameter twice that of the drilled and tapped hole 3 therein was inserted into pocket 1 and held in place by a suitable epoxy glue 4.
This attempt was more successful than the previous attempt. However, it was found that there were failures resulting from rapid changes in operating temperature and poor heat transfer through the glue. Also, because of the configurations of the panels, there were places where it was not possible to put a replacement plug.
Another problem with the plug attempt is that air will often be trapped between the plug and the panel. Later, when the panel is reassembled and in the vacuum environment of the AVLIS process, the trapped air will leak out, making it harder to pull the necessary vacuum in the process. In an effort to eliminate this problem, a relief passage 5 has been drilled into the bottom of pocket 1. This procedure increased the cost of repair and, again, could not be used in many locations of the panels.